1. Field of the Invention
The invention relates generally to the field of indicator reagents and their use in diagnostic assays. More particularly, the present invention relates to the use of organic polymer latex particles as the label components for such indicator reagents, which are especially advantageous in immunoassays.
2. Description of Related Art
Various analytical procedures are commonly used in diagnostic assays to determine the presence and/or amount of substances of interest or clinical significance in test samples, such as body fluids. These clinically significant or interesting substances are commonly referred to as analytes. Diagnostic assays have become an indispensable means for detecting analytes in test samples by using the mutual reaction between the analyte and a specific binding member, as typified by the immunoreaction between an antigen and the antibody to that antigen.
In detecting immunoreactions, use has been made of tags or labels composed of a traceable substance that is attached to a specific binding member, such as an antibody, which in turn binds to the analyte. The detection of the labeled antibody/analyte complex, or of the labeled antibody which remains unbound, is used to indicate the presence or amount of the analyte in the test sample.
Two commonly used immunoreaction techniques are the radioimmunoassay (RIA) and the enzyme immunoassay (EIA), both of which employ a labeled specific binding member, i.e., indicator reagent. The RIA uses a radioactive isotope as the traceable substance attached to a specific binding member. Because the radioactive isotope can be detected in very small amounts, it can be used to detect or quantitate small amounts of analyte. There are, however, a number of substantial drawbacks associated with the RIA. These drawbacks include the special facilities and extreme caution that are required in handling radioactive materials, the high costs of such reagents and their unique disposal requirements.
The EIA uses an enzyme as the label attached to a specific binding member, and enzymatic activity is used to detect the immunoreaction. While the EIA does not have the same disadvantages as the RIA, many EIA techniques require the addition of substrate materials to elicit the detectable enzyme reaction. In addition, enzymes are temperature sensitive and generally have decreased stability at elevated temperatures such as 45.degree. C. shipping temperatures.
More recently, assay techniques using metallic sol particles as labels have been developed. In these techniques, a metal (e.g., gold, silver, platinum), a metal compound, or a nonmetallic substance coated with a metal or a metal compound is used to form an aqueous dispersion of particles. The specific binding member to be labeled is coated onto the metal sol particles by adsorption. The metal sol particles have the advantage of producing a signal that is visually detectable as well as measurable by an instrument, but despite their utility, the inorganic particles have several disadvantages. The metallic particles have a limited color intensity, and therefore, increased amounts of such labels are needed in an assay for ease of detection. In addition, the surfaces of inorganic metallic colloid particles, such as gold, do not readily accept the covalent attachment of specific binding members. Thus, during use in a binding assay, care must be taken so that the adsorbed specific binding members are not removed from the inorganic particles through the combination of displacement by other proteins or surface active agents and the shear forces which accompany washing.
Particles used as labels in immunoassay reagents have also been formed from dye polymers. In this group, dye molecules, i.e., chromogenic monomers, are polymerized to form a colored polymer particle. The dye particles can then be linked to a specific binding member for use in the assay. Examples of such dyes include Congo red, Trypan blue and Lissamine blue.